In order to attain cost-saving efficiency and productivity in metal extrusion technologies, it is important to achieve thermal alignment of the extrusion press. Thermal alignment is the control and maintenance of optimal running temperature of the various extrusion press components. It ensures that the flow of extrudable material is uniform and enables the press operator to press at maximum speed, with less waste. A number of factors must be considered when assessing the thermal alignment of an extrusion press. For example, the billet of extrudable material must be completely at the optimum operating temperature in order to assure uniform flow rates over the cross-sectional area of the billet. The temperature of the liner in the extrusion container must also serve to preserve and not interfere with the temperature profile of the billet contained therein (i.e. uniform or tapered).
Achieving thermal alignment is generally a challenge to a press operator. During extrusion, the top of the extrusion press container usually becomes hotter than the bottom. Although conduction is the principal method of heat transfer within the container, radiant heat lost from the bottom surface of the container rises inside the container housing, leading to an increase in temperature at the top. As the front and rear of the container are generally exposed, they will lose more heat than the center. This may result in the center section of the container being hotter than the ends. As well, the temperature at the die end of the container tends to be slightly higher compared to the rain end, as the billet heats it for a longer period of time. These temperature variations in the container affect the temperature of the finer contained therein, this in turn affecting the temperature of the billet of extrudable material. While the total flow of extrudable material from the press depends solely on the speed of the ram, flow rates from hotter sections of the billet will be faster compared to flow rates from cooler sections. The run-out variance across the cross-sectional profile of a billet can be as great as 1% for every 5° C. difference in temperature. This can adversely affect the shape of the profile of the extruded product.
In view of these multiple interactions between the container, the liner and the billet, the overall extrusion system requires a dynamic means to control and maintain temperature and preselected temperature profiles.
One method known in the art is to provide heating elements in the container housing, surrounding the mantle. Examples of this technology include U.S. Pat. Nos. 3,385,953 and 3,531,624 which teach the use of multiple arcuate heating coils. Another example is U.S. Pat. No. 3,113,676 which teaches a more complete circumferential wrapping about the mantle. This means of heating an extrusion press container, which is based largely on convection, presents certain challenges. First, since the heating elements are located around the container, in essence as a “blanket”, they are considerably distant from the temperature sensors or thermocouples generally located near the liner. In a large container, this distance could exceed 30 cm. As a result, in addition to losing a considerable amount of heat to the container holder and surrounding environment, the response time to measured temperature conditions is unavoidably slow. Second, the heating elements used generally have a sheath temperature of 705 to 760° C. In maintaining a temperature of 425 to 480° C. at the liner, the temperature near the surface of the mantle can easily reach more than 705° C. This is well in excess of the annealing temperature of 540° C. for the 4340 steel generally used to manufacture this component. These factors increase the risk of annealing and softening of the mantle, leading to a deformation of the liner and loss of physical alignment of the extrusion press. The overheating and softening of the mantle also increases the risk of liner fracture under full ram pressure. In addition, annealing of the mantle and deformation of the liner may lead to the accumulation of impurities, with subsequent contamination of the product. In extreme cases, mantle fracture is also a possibility. Furthermore, if the outside of the container becomes considerably hotter than the liner, the interference fit between the liner and the mantle may be adversely affected. This would result in the failure of the shrink fit causing the liner to loosen and slip.
Another method of controlling the temperature of the container is to position the heat source inside the container itself. A variety of configurations for this technology are known. These configurations include longitudinally oriented elements (U.S. Pat. Nos. 2,075,622 and 3,161,756), spirally oriented elements (U.S. Pat. No. 2,792,482), circumferentially oriented elements (U.S. Pat. No. 2,820,132) as well as radially oriented elements (U.S. Pat. No. 2,853,590). Although this method is an improvement compared to the “blanket” heaters discussed above, conductive and radiant heat is still being applied to the core of the mantle, with the temperature sensors being spatially distant on the liner. Depending on the location of the heating elements in the container, the response time to temperature changes in the liner can be far from immediate.
In general, when the extrusion press is run continuously, little more than minor temperature adjustments should be necessary to maintain thermal alignment of the press. When the press has been stopped, however, the container must be preheated to minimize “chilling”, or thermal shock to the billet on start-up. Preheating the container in a manner that is both quick and efficient, in a manner that does not adversely affect the container itself, as well as maintaining operating temperature during brief stops can be difficult. In general, the operator should aim to reduce the likelihood of thermal fatigue in the container by implementing means to minimize the temperature difference between the mantle and liner during both extrusion and down periods.